the bet load but it's hard. People like to bet on their home team... So if you're a sports book on LA you get a ton of bets on the Lakers. It's really hard to balance that out.
Books actually sell their bets to other books to try to balance things out. So let's say the Lakers are playing the 76ers. A book in Philly may offload a chunk of their Philly bets to a sports book in LA who wants to try and balance their bets.
But balancing things out doesn't always work and the books can lose big.
Generally speaking they have a good business model, but certainly a book can lose money in any given year.